1. Field of the Invention
The invention relates to a device for performing a thermal measurement method, from which the gas pressure in an evacuated thermal insulating board can be determined where this latter has an insulating core enclosed in a film. Likewise, the invention centers on a method for measuring the gas pressure inside an evacuated thermal insulating board.
2. Description of the Related Art
Evacuated thermal insulation plates or vacuum panels reach a high insulation effect with the smallest insulation thickness. Usually, they consist of an evacuable, porous core material of low thermal conductivity, and of a vacuum sealed covering shell, the latter composed, for example, of a metallized high-barrier synthetic film. For applications that require longer life spans, in building structures for example, an insulating core of microporous silica has been demonstrated to have advantages. For applications demanding shorter periods of use, the insulating core can also be composed of either an open-pored foam consisting of polyurethane and/or polystyrene, or of fiberglass. Because of the extremely small size of their pores, with diameters of less than one half of a micrometer, a vacuum from 1 to 10 mbar in the core materials is sufficient to practically eliminate the thermal conductivity of the air in micro-porous cores. At these pressure levels, a heat conductivity between 0.004 W/(m*K) and 0.005 W/(m*K) is achieved. An increase of gas pressure to 100 mbars allows the heat conductivity to rise only to around 0.008 W/(m*K) whereas the thermal conductivity averages 0.020 W/(m*K) at normal air pressure of 1000 mbar. Because of the larger pore dimensions of open-pored foams or fiberglass materials, gas pressure levels must lie between 0.01 and 1 mbar in order to significantly suppress the heat conductivity of the air.
The performance and efficient function of the vacuum insulation can be determined by the level of thermal conductivity or by the interior gas pressure. A measurement of thermal conductivity at individual vacuum insulating boards is possible with the normal stationary board measuring method. While such methods enjoy a relative precision, lying below 5% in relation to the absolute thermal conductivity, they are nevertheless extremely time consuming. On the other hand, hitherto available non-stationary techniques, which involve impressing a board with a heat or temperature pulse and measuring such values like temperature or heat flow in a time dependent manner, are faster than the stationary techniques, but they tend to be inexact and prone to false results. Moreover, this is because the measured values depend only on the square root of the thermal conductivity of the insulating core.
The performance of a micro-porous vacuum insulating board can be judged most precisely by means of the level of the interior gas pressure. The initial gas pressure of a newly produced vacuum insulating board typically averages between 1 and 5 mbar. When a suitable high barrier film is used, the interior gas pressure should increase at a rate of no more than 0.2 to 2 mbar per year. So calculated, one can expect to arrive at a doubling of the thermal conductivity in micro-porous thermal insulating boards at the earliest in fifty years. Data about gas pressure levels—as the case may be, the rate of increase over time in a built-in state—is significant for assuring the quality of the vacuum insulating board.
The interior gas pressure of the vacuum panel can be controlled by placing the test object in a vacuum chamber and decreasing the pressure until the film rises noticeably from the insulating core. In this case, the interior pressure of the panel becomes just greater than the gas pressure in the vacuum chamber. This method requires a vacuum insulating chamber that is at least the size of the panels. Moreover, the gas pressure of a vacuum insulating board that has been committed or already built into an insulating object can no longer be determined, so other methods must be found for these purposes.
According to the above observations, the problem inducing the present invention is to make it possible to determine the gas pressure in vacuum insulating boards which are already committed and/or already built into insulating objects; with a high precision, if possible. In such a test, the covering may not be damaged. The technical complexity of the measurement should be held as low as possible; in particular, the time required for it should be kept at a minimum. Furthermore, wherever possible every attention must be paid that the production costs of the vacuum panels are not significantly raised.